1. Field of the Invention
This invention relates to flow measurement of abrasive fluids and more particularly to a flow meter suitable for use in a drilling tubular.
2. Description of the Related Art
Drilling systems for oil and gas production commonly include a drilling fluid flowing down a tubular member in a borehole and back up to the surface. The drilling fluid serves several purposes known in the art, such as removing cuttings from the wellbore. The drilling fluid is a mixture of a liquid with solid particles in suspension. As is known in the art, some of the solids may be added to the liquid to control the viscosity and gel strength of the liquid, for example, to maintain the cuttings in suspension in the drilling fluid. Other solids, such as barite, may be added to control the hydrostatic pressure near the bottom of the wellbore, so as to control formation fluid flow into the wellbore. The drilling fluid inside the tubular member may be at relatively high pressure and be at flow rates in the range of 100 gpm to over 1000 gpm. Measurement of the flow rate of the drilling fluid in such a situation is problematic.
Common obstruction type flow meters, such as orifice plates, result in unacceptable pressure losses across the flow metering section. FIG. 1 depicts a common venturi type meter where the flow is accelerated through the gradually reducing conically tapered nozzle section to the minimum area at the throat section. The accelerated flow has a related static pressure drop between the entrance pressure port and the throat pressure port with the pressure drop being determinable from Bernoulli's equation, well known to those skilled in the art. The fluid flow expands and diffuses in the gradually tapered diffuser section such that as much as 75% of the pressure drop is recovered. The pressure differential between the entrance and the throat may be detected by a differential pressure transducer known in the art. The system as described in FIG. 1, however, has serious drawbacks when used in a drilling fluid flow system. For example, when the flow meter is located in a vertical orientation, the solids in the drilling fluid have a tendency to settle out in and clog the fluid lines leading from the pressure ports to the differential pressure sensor. In addition, the sensing pressure ports create a flow disturbance that leads to locally accelerated erosion caused by the particles in the drilling fluid impinging on the port.
Thus, there is a demonstrated need for a flow meter that is suitable for use in abrasive fluid flow and is resistant to clogging and erosion.